Optimizing conditions for radiolabelling DTPA-bombesin analogues with In-111 at high specific activity

Abstract

In designing radiometal-based peptides, an important factor to consider is the specific activity (SA) of the molecule. Low SA can compromise the uptake of the tracer in the tissue of interest in vivo and lead to physiological responses due to the presence of the cold peptide in the organism. However, very high SA can cause radiolysis of the compound, resulting in undesirable impurities. In this study, parameters influencing the kinetics of labelling of a DTPA-bombesin analogue (BETG5) with 111In were investigated and conditions were optimized to obtain the highest achievable SA. The effects of peptide mass, 111In activity, temperature and time of reaction in radiolabelling yield and peptide integrity were systematically categorized applying ITLC-SG and HPLC as chromatography techniques. Kinetics of 111In-labelling was optimal at pH 4.5, room temperature and 15 min of incubation. Higher radiochemical purities were obtained when 10 μg of BETG5 reacted with 37–555 MBq of radiometal (maximum SA 111 GBq/ μmol). 111In-BETG5 integrity analysis suggested that oxidation does not depend on radiolabelling conditions and can be avoided by the addition of methionine in radiolabelling medium. These optimized conditions will be applied to produce high specific activity DTPA-bombesin analogues for preclinical studies in healthy and tumour mice.